J.M. Henriques

Structural, electronic and optical properties of ilmenite and perovskite CdSnO3 from DFT calculations

CdSnO(3) ilmenite and perovskite crystals were investigated using both the local density and generalized gradient approximations, LDA and GGA, respectively, of the density functional theory (DFT). The electronic band structures, densities of states, dielectric functions, optical absorption and reflectivity spectra related to electronic transitions were obtained, as well as the infrared absorption spectra after computing the vibrational modes of the crystals at q = 0. Dielectric optical permittivities and polarizabilities at ω = 0 and ∞ were also calculated. The results show that GGA-optimized geometries are more accurate than LDA ones, and the Kohn-Sham band structures obtained for the CdSnO(3) polymorphs confirm that ilmenite has an indirect band gap, while perovskite has a direct band gap, both being semiconductors. Effective masses for both crystals are obtained for the first time, being highly isotropic for electrons and anisotropic for holes. The optical properties reveal a very small degree of anisotropy of both crystals with respect to different polarization planes of incident light. The phonon calculation at q = 0 for perovskite CdSnO(3) does not show any imaginary frequencies, in contrast to a previous report suggesting the existence of a more stable crystal of perovskite CdSnO(3) with ferroelectric properties.

Triclinic CdSiO3 structural, electronic, and optical properties from first principles calculations

Quantum ab initio simulations were carried out to study the CdSiO3 triclinic crystal. Unit cell parameters and atomic positions were optimized to find a minimum total energy within the density functional theory (DFT) formalism in both the local density and generalized gradient approximations, LDA and GGA, respectively. Analysis of the Kohn?Sham electronic band structure shows that there are two very close indirect band gaps Eg(Z ? ?) = 2.57?eV (2.79?eV) and Eg(Q ? ?) = 2.59?eV (2.81?eV) for the GGA-PBE (LDA-CAPZ) computations, and a direct band gap Eg(? ? ?) = 2.57, 2.63?eV (2.85?eV). Effective masses for holes and electrons were estimated by parabolic fitting along different directions at the valence band maximum and conduction band minimum, and they are very anisotropic. A comparison with previously reported data for triclinic CaSiO3 (wollastonite) using the LDA-CAPZ exchange-correlation functional reveals that the substitution of calcium by cadmium changes the localization of the valence band maximum in reciprocal space and decreases the band gap energies. Optical properties (dielectric function, optical absorption) for incident light polarized along different crystalline planes were computed, the optical absorption for incident light with polarization along the 0?1?0 crystalline plane being the smallest for energies near the main band gap due to the spatial disposition of the SiO4 tetrahedra and CdO6 octahedra chains that build up the structure of triclinic CdSiO3.

First-principles calculations of structural, electronic and optical properties of orthorhombic CaPbO3

Calculations within the density functional theory approach were performed to obtain structural parameters, electronic band structure, carrier effective masses and optical absorption spectra in orthorhombic CaPbO3. Both local density and generalized gradient approximations, LDA and GGA, respectively, were considered. A comparison reveals good agreement of the calculated lattice parameters with experimental results. A direct Γ → Γ one-electron energy band gap of 0.84 eV (0.94 eV) was obtained within the GGA (LDA) level of calculation, in contrast to a previous interpretation of experimental data pointing to a gap of only 0.43 eV. Comparting our results with band gap energies previously obtained for CaXO3 crystals (X = C in calcite, X = Si in wollastonite and X = Ge,Sn,Pb in the orthorhombic phase), we note that the energy gap oscillates, but with an overall trend to decrease, as the atomic number of the X atomic species increases.